Sky and Telescope - April 2018 - 32

Hydra I Cluster

n clear April evenings, the splashy winter constellations slide toward the western horizon, and deep-sky
enthusiasts are itching to dive into the galaxy-rich
ﬁelds of Virgo, Leo, and Coma Berenices. These constellations, along with Canes Venatici and Ursa Major, headline
32 of the 40 Messier galaxies. But farther south and less
explored, the winding constellation of Hydra, the Water
Snake, uncoils across 100° of the sky and contains its own
bonanza of galaxies.
At a distance of 165 million light-years, the Hydra I Cluster
(also known as Abell 1060) is one of the nearest rich galaxy
clusters beyond the Virgo Cluster. Physically, the two clusters
display a striking resemblance. Hydra I is three times as distant as the Virgo cluster, but its apparent size is 1/3 as large. So
their linear dimensions are nearly the same. And each cluster
contains 50 galaxies within two magnitudes of their dominant members, M87 in Virgo and NGC 3311 in Hydra, both
supergiant ellipticals with comparable absolute magnitudes.
Redshift surveys have demonstrated that the cluster is
remarkably isolated in space, with no foreground galaxies and
a huge void in the background out to a distance of 400 million light-years. This allows for easy identiﬁcation of cluster
members to fainter magnitudes based on redshift. The NASAIPAC Extragalactic Database lists upwards of 350 known
members in a 5° region.
John Herschel discovered nine of the central galaxies that
carry NGC designations in March 1835 and 1836 during his
survey of the southern sky from the Cape of Good Hope. In
1835, he observed the cluster on three consecutive sweeps
q HYDRA I FINDER CHART Find the cluster nestled between Mu, Nu,
and Xi Hydrae and Alpha Antliae. Two outliers, NGC 3313 and NGC
3393, lie slightly north and northeast, respectively, of the cluster core.

11h 00m

10h 30m

CRATER

Alphard

SEXTANS

9h 30m

10h 00m a

-10°

g
+

b
a

_
-20°

Star magnitudes

HYDRA
44

2
3
4
5
6
7

3313
3393
Hydra I

-30°

f
A PR I L 2 018 * SK Y & TELESCOPE

◗ Abell galaxy clusters are often abbreviated as
"ACO [Number]" for the Abell-Corwin-Olowin 1989
update, so another name for Abell 1060 is ACO 1060.

ANTLIA

(slowly swinging the scope up and down several degrees while
pointing due south) with his 18-inch speculum reﬂector in
an attempt to accurately determine positions. But timing the
closely spaced meridian transits while recording the altitude
was difﬁcult as the ﬂurry of galaxies drifted by. As a solution,
he made carefully drawn sketches in both years but realized
"in each diagram only 7 were seen and laid down, yet there
are in reality at least 9 in the whole group."
Hydra I just made the southern cut-off limit (-27° declination) of George Abell's 1958 catalog of 2,712 rich galaxy
clusters, yet it's still accessible to mid-northern observers.
And a large aperture isn't a requirement; several 11.5- to
12.5-magnitude members are within reach of a 6- to 8-inch
scope. But to minimize the effects of poor seeing and reduced
transparency close to the horizon, make your observation
when the cluster is highest near the meridian - about 11:00
p.m. at the beginning of April and an hour earlier around the
new Moon at mid-month.
The core of the Hydra I Cluster forms a wide isosceles triangle with 4th-magnitude Alpha (α) Antliae and 5th-magnitude 44 Hya. The cluster is slightly under 4° from either star,
lying just to the east of a line connecting the two stars. The
main drag of the cluster's core is easy to follow as it forms
a 20′ chain passing between 4.9-magnitude HD 92036 and
6.7-magnitude HD 91964. You'll ﬁnd half a dozen brighter
NGC galaxies, but the glare from these stars can be an
annoyance, so use medium or high power and isolate them
outside the eyepiece ﬁeld whenever possible. My descriptions
are based on observations through a 13.1-inch reﬂector using
166× and 214× from dark sites in both northern California
and tropical Costa Rica.
Let's start with NGC 3309 and NGC 3311, which form
an impressive pair at the heart of the cluster. NGC 3309,
an 11.6-magnitude elliptical, is strongly condensed with a
conspicuous 40″ core encased in a slightly elongated 1.2′
halo leaning northeast. NGC 3311, just 1.7′ east-southeast,
is similar in size and magnitude but more broadly concentrated with no distinct nucleus. Using averted vision, its
tenuous outer envelope grows to nearly 2′ in diameter with
a 13.5-magnitude star sandwiched between the halos of the
two elliptical galaxies.
NGC 3311 is a colossal cD-type (central dominant) galaxy
with a distended diffuse halo composed of stars and debris
cannibalized from nearby satellite galaxies. It commands a
swarm of roughly 16,000 globular clusters, one of the largest

Sky and Telescope - April 2018

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